Commercial investment casting typically involves use of a shell mold formed by the "lost wax process" wherein a fugitive pattern of the article to be cast is repeatedly dipped in ceramic slurry and stuccoed to build up the shell mold. The pattern is then removed, and the green mold fired to develop adequate strength for casting. Such shell molds provide good control over casting chemistry, dimensions, heat removal/solidification rates, and cooling stresses. These factors are especially important to directionally solidified and single crystal (DS/SC) castings. To date, most DS/SC castings are made of nickel-based superalloys. Even with various mold materials and casting methods, castings sometimes have hot tearing or hot cracking problems due to excessive shell strength following melt solidification. Furthermore, if a DS/SC casting is excessively stressed during mold removal, it may be subject to cracks or grain recrystallization.
Various mold compositions have been proposed for use in high temperature DS/SC casting processes.
Greskovich (U.S. Pat. No. 4,188,450) and Takayanagi (U.S. Pat. No. 4,664,172) disclose refractory mold compositions of alumina with a silica binder, which form mullite on firing above 1400.degree. C. These molds have only fair sag resistance at intermediate temperatures (1200.degree.-1400.degree. C.) encountered during the DS/SC casting process due to binder softening before the mullite forms. Their excellent hot strength is retained upon cooling from the casting cycle. This can damage the casting on cooling or during shell removal. Furthermore, the silica or mullite is reactive with several common alloying elements. Svec et al. (U.S. Pat. No. 4,247,333) describes removal of the silica from such molds by firing in a reducing atmosphere, but the method disclosed is slow and costly, and does not address the problems of excessive mold strength after casting.
Fassler et al. (U.S. Pat. No. 3,933,190) disclose an all-alumina shell mold bonded by aluminum oxychloride. This shell mold has low green strength, is not suitable for autoclave dewaxing, and requires a high firing temperature prior to use. This shell mold also becomes excessively strong, due to sintering, during exposure to DS/SC casting temperatures.
Feagin (U.S. Pat. Nos. 4,196,769 and 4,216,815) describes a shell mold material with an alumina binder, producing a mold green strength of only about 50% of that of conventional shell molds. Shell mold strength was found to increase with firing temperature such that these shell molds have exhibited excessively high strength following DS/SC casting, possibly leading to casting defects. Shell molds made by this art have not been found to be autoclavable, since the binder swells on exposure to steam.
Mills (U.S. Pat. No. 4,617,977) describes an injection or transfer shaped mold body that is fired to an elevated temperature. This process can yield dimensionally precise, thin walled casting molds. However, the process requires more tooling than a slurry dipped/stuccoed shell mold process. Also, time and equipment intensive firing steps are required. The resulting fine grained mold can have extensive firing shrinkage, and can be too strong to be removed effectively after casting.
In general, nickel aluminide (NiAl) is an intermetallic compound with potential for use in gas turbine engines. In particular, turbine blades and vanes of this material offer significant opportunities for improving gas turbine engine performance. NiAl based alloys exhibit lower densities, higher thermal conductivities, higher melting temperatures, and greater oxidation resistance than the most advanced directionally solidified/single (DS/SC) crystal nickel based superalloys. The NiAl alloys exhibit high melting points which may allow their use at higher operating temperatures than the aforementioned conventional nickel based superalloys.
However, even in alloyed compositions, NiAl has limited ductility, and near zero ductility below the ductile to brittle transition temperature (DBTT) of about 1000.degree. F. Although it is desirable to make net shape single crystal NiAl turbine components by investment casting, conventional mold technology is limited in at least four respects with respect to this material: 1) the molds do not retain dimensional stability at the high (&gt;3000.degree. F.) casting temperatures required for NiAl, 2) the molds have excessive reaction with the NiAl melt and this results in contamination of the casting, 3) the molds may cause hot tearing/hot cracking due to excessive mold strength during cooling, and 4) the molds are difficult to remove without damaging the casting.
For example, current silica bonded ceramic shell mold technology has been unable to yield cast NiAl based alloys without hot tearing and cracking of the casting. In particular, as a result of its high melting temperature (about 2990.degree. F.), low ductility below about 1000.degree. F., and low strength relative to the conventional silica bonded zircon shell mold systems, DS/SC castings of NiAl alloys hot tear and crack during solidification and cooling in the ceramic mold. Moreover, slumping, bulging and even melting of conventional silica bonded zircon molds has occurred during investment casting of NiAl alloys when the mold is exposed to temperatures above 2900.degree. F. for a prolonged time. This behavior is attributed to the relatively low creep strength and of such molds at the casting temperatures involved. Mold slumping, bulging, or melting produces castings which are not dimensionally accurate. Furthermore, prior art workers are concerned about silicon pick-up in the casting using this conventional mold technology.
With respect to current silica bonded alumina shell mold systems, some of the silica binder volatizes during exposure at NiAl casting temperatures and contaminates the furnace atmosphere with silicon. Moreover, silica bonded alumina shell molds become sintered at NiAl alloy casting temperatures and, as a result, become much stronger than the casting. Castings hot tear or crack during cooling in the mold as a result.
Investment casting of NiAl based alloys in conventional silica bonded zircon and alumina mold systems is not viable for producing turbine airfoils (blades and vanes) as a result of the cracking or hot tearing observed. Likewise, sintering of alumina or zirconia bonded shell molds can cause high strengths which lead to cracks in the NiAl castings. While these types of shell molds do not have the problems of slump, limited refractoriness, or contamination of the silica bonded molds, they still present a cracking problem. Also, the green strength of alumina or zirconia bonded shell molds produced by the prior art have been deficient. Moreover, they are easily damaged in handling or from wax expansion stresses, and some of these mold systems can not be autoclave dewaxed. Such disadvantages of alumina or zirconia bonded shell molds detracts from their use as a production mold system.
Various mold systems have been proposed for reducing mold strength after casting solidification.
Watts (U.S. Pat. Nos. 4,533,394 and 4,689,081) discloses an artificial stucco material, comprising a refractory powder and an organic binder. The binder is burned out during mold firing. While this approach would aid shell removal from the solidified casting, it would increase mold shrinkage and reduce mold strength at DS/SC casting temperatures. It also would require extra processing steps in manufacturing the artificial stucco.
Klug et al. (U.S. Pat. Nos. 4,164,424; 4,191,721 and 4,221,748) describe a porous core or mold material produced from a mixture of alumina, an organic binder, and a reactive fugitive filler. These injection molding mixtures are not suitable for shell mold fabrication. The shaped mixtures require carefully controlled firing conditions and have high firing shrinkage. Their porous microstructure is made by reduction and vapor transport of the alumina during sintering. The patent points out that "unbound carbon" should be removed prior to eutectic or superalloy casting.
Lirones (U.S. Pat. No. 3,239,897) discloses a shell mold composition which includes ceramic powders and stuccos, bonded by colloidal graphite. The colloidal graphite could impart easier removal of the mold from the casting and cleaning of the casting than the sodium silicate bonded molds. However, the graphite binder does not exhibit green strength comparable to the silicate binders. The mold is burned out under special conditions (reducing atmosphere). Finally, the use of zircon or silica refractories with a graphite binder would result in significant mold/melt reactions if heated to DS/SC casting temperatures.
Manginelli (U.S. Pat. No. 3,362,463) describes the use of solid or hollow "globules" to reduce the weight and cost of investment molds in flask or shell configurations for making equiaxed grain castings. In the intended application for making equiaxed grain castings, it is unnecessary to preheat the mold to high (&gt;2200.degree. F.) temperatures. Consequently, the patent is not faced with problems of excessive mold sintering, shrinkage, or distortion which accompany higher preheat temperatures. The patent indicates that the large fraction of porosity helps equiaxed castings cool slowly. This would be a detriment in the practice of this invention wherein directionally solidified/single crystal (DS/SC) castings are made. The patent also mentions that "shake out" of the mold with hand or pneumatic hammers would be facilitated; however, the mold is not designed to "collapse" at low stress due to cooling of a solidified DS/SC casting.
Noting the aforementioned limitations of conventional mold technology and its unsuitableness for SC investment casting of brittle intermetallic alloys, it is an object of the present invention to provide mold technology which overcomes these difficulties and is useful in producing sound, crack-free castings, such as DS and SC castings, of intermetallic alloys, superalloys and low ductility materials other than intermetallic alloys and superalloys.
It is another object of the present invention to provide a casting mold, and method of making same, which is useful for investment casting of nickel or titanium aluminide alloys as well as other high melting point, low ductility metals and alloys without hot tearing or cracking during solidification and cooling of the casting in the mold.
It is another object of the present invention to provide a casting mold, and method of making same, for casting nickel aluminide alloys as well as other high melting point and/or low ductility metals and alloys wherein a region of the mold is selectively crushed or deformed as necessary as the casting solidifies and cools in the mold below the metal/alloy ductile-to-brittle transition temperature so as to avoid hot tearing or cracking of the casting by compressive mold stresses.
It is still another object of the present invention to provide a method of casting nickel aluminide alloys as well as other high melting point and/or low ductility metals and alloys without hot tearing or cracking during solidification and cooling of the casting in a ceramic mold.
One particular object of this invention is to provide a shell mold which is capable of enduring a 2700.degree. F+ DS/SC casting cycle without excessive shrinkage or distortion while exhibiting enough mold strength at temperature to hold the molten alloy.
Another particular object is to provide a shell mold which does not increase in strength during a DS/SC casting cycle.
Another particular object is to provide a shell mold which does not induce hot tearing/cracking to castings during cooling from the casting process.
Another particular object is to provide a shell mold which is easily removed from a casting without mechanically or chemically damaging the casting.
Another particular object is to provide a shell mold with sufficient green strength to be handled, dewaxed, and/or fired using production techniques.
Another particular object is to provide a shell mold which is substantially free of silica binder, is amendable to autoclave dewaxing, and exhibits sufficient post-dewax strength for handling and casting, without a high temperature (&gt;2000.degree. F.) sintering cycle.